A VLAN is a local area network that maps workstations or nodes on a basis other than geographic location (for example, by department, type of user, or primary application). VLANs are used to regulate the access of information among nodes that reside in different multiple networks. Although these networks are physically connected, logically they are separate.
FIG. 1 illustrates an example of prior art VLANs. In this example, there are three Ethernet LAN segments 101, 102 and 103 connected with each other through network 100 using switches 106, 107 and 108 respectively. Each of the switches 106, 107, 108 performs the transfer of frames from one Ethernet segment to another Ethernet segment. The Ethernet segment 101 includes nodes 105 and 110. The Ethernet segment 102 includes nodes 115 and 120. The Ethernet segment 103 includes nodes 125 and 130. Each of the nodes in the Ethernet segments 101, 102, 103 may be a workstation such as a personal computer.
VLAN traffic is generated by multiple different applications. Some of these applications are mission critical applications and demand higher priority of service than other applications. VLANs can be implemented in an Ethernet network by inserting a VLAN tag into an Ethernet frame. Originally, an Ethernet frame is defined to have a minimum frame size of 64 bytes and a maximum frame size of 1518 bytes. The maximum allowable frame size is extended by four (4) bytes to 1522 bytes to allow insertion of the VLAN tag. Of these four (4) bytes, the last two bytes or 16 bits are used as follows: the first three (3) bits are used as a priority field to assign a priority level to the Ethernet frame, the next one (1) bit is used to indicate the presence of a routing information field (RIF), the last 12 bits are used as a VLAN identifier (VID) field that uniquely identifies the VLANs to which the Ethernet frame belongs. Each VLAN gets a unique network number or identifier (using the VID field).
Referring to FIG. 1, the nodes 105 and 115 (nodes A and C) may be configured to be in a first VLAN. Similarly, the nodes 110, 120, 125 and 130 (or nodes B, D, E, and F) may be configured to be in a second VLAN. A node can belong to more than one VLAN. For example, the node 115 (or node C) may be configured to be in both the first VLAN and the second VLAN. As illustrated, the nodes in the first VLAN and in the second VLAN may be connected to different Ethernet segments. Thus, a port on the switch 106 may receive frames from the first VLAN or from the second VLAN. The switch delivers the frames based on the VID in the VLAN tag of each packet. When the node 110 (or node B) sends a broadcast message, frames associated with the broadcast message are sent only to those nodes belonging to the second VLAN (i.e., nodes D, E and F), even though the frames may cross the switches 106, 107, and 108. Each switch reads the incoming frames and, according to how the addressing (e.g., port assignment) of the network is defined, identifies each VLAN.
The network 100 may be a connection-oriented network such as, for example, an Asynchronous Transfer Mode (ATM) network and Multi-Protocol Label Switching (MPLS) network. In a connection-oriented network, there may be different type of services (e.g., bandwidth, jitter, delay, etc.) to handle different types of traffic (e.g., voice, video, data, etc.). The type of service is established at the time that the connection is made. For example, in the ATM network, the ATM Forum has defined four quality-of-service types of service that are architected to handle the different types of traffic. These types of service include Constant Bit Rate (CBR), Variable Bit Rate (VBR), Unspecified Bit Rate (UBR), Guaranteed Frame Rate (GFR), and Available Bit Rate (ABR).
CBR is a reserved bandwidth service. A contract is established between the network and the end station. The end station provides the network with parameters describing the traffic for that specific connection at call setup time. The network, in turn, allocates resources that match the parameters. VBR is also a reserved bandwidth service. The network allocates resources to the end station at call setup time in response to the traffic parameters requested by the end station. In the case of VBR, however, in addition to a peak rate, a sustainable rate and a maximum burst size are established. The sustainable rate is the upper limit of the average rate, and the maximum burst rate limits the duration of cell transmission at peak rate. GFR provides a rate guarantee for application traffic across networks. UBR is a non-reserved bandwidth service. The cell loss is unspecified, which means that the network is not required to provide resources for a proposed UBR connection. ABR is a mix of reserved and non-reserved bandwidth service. Periodically, a connection polls the network and, based upon the feedback it receives, adjusts its transmission rate.
As described above, the three bits priority field in the VLAN tag is used as a priority field to assign a priority level to the Ethernet frame. When, however, these Ethernet frames are sent across a connection-oriented network such as the ATM network 100, one connection is used. The quality of service of this connection may be CBR, VBR, GFR, UBR, or ABR. Using the above example, all Ethernet frames transmitted by the node 110 to the node 125 are transferred from the switch 106 to the switch 108 using the same connection having the same ATM quality of service. A connection is identified by its VPI/VCI. VPI is a virtual path identifier that identifies a path that an ATM cell will take. VCI is a virtual channel identifier that identifies a channel within the path that the ATM cell will take to reach its destination.
Because the same connection is used, the priority values in the VLAN tags of the Ethernet frames are not considered. As such, Ethernet frames having higher priority are treated the same by the ATM network 100 as Ethernet frames having lower priority.